In the last 4 years of funding we have defined the conditions under which T cells obviate the requirement for costimulatory signaling during activation, and mediate graft rejection in murine models of transplantation despite treatment with reagents that profoundly Inhibit costimulatory T cells to T cells. Specifically, our research has shown that the presence of high naive CD4'^ or CDS* T cell precursor frequency or pre- existence of donor-reactive memory T cells render recipients refractory to the salutary effects of costimulation blockade. These findings have been based on the use of TCR transgenic models which allow the use of a fixed, monoclonal T cell population in order to control for the effects of altered TCR affinity for antigen, among other variables present In endogenous, polyclonal, polyantigen-specific populations. However, during the course of our studies we have begun to Investigate the role of TCR affinity for pMHC complexes during graft rejection, and how this parameter fundamentally impacts the magnitude and character of the donor-reactive T cell response during graft rejection or survival. After establishing a novel transgenic system In which graft specific T cells are primed by ligands of increasing affinity for the TCR, we have demonstrated In preliminary studies that graft-specific TCR affinity profoundly impacts the effector phenotype of responding T cells, and more Importantly, critically influences their susceptibility to costimulatlon blockade- Induced prolongation in graft survival. Therefore, experiments outlined In this proposal will endeavor to elucidate the role of TCR affinity in precipitating graft rejection, and to assess the mechanisms underlying the ability of cells stimulated with ligands of varying potencies to be tolerized using costlmulatlon blockade. These studies are of great significance because they will allow us to begin to unravel the nature of the alloreactive T cell response, and define a threshold for TCR cross-reactivity above which clinical manifestations of T cell alio- crossreactlvity will occur.